It is a conventional feature of automatic machines for moving materials, particularly sheet-like materials, that the material is handled using suction devices connected to air lines of a pneumatic system. Systems of this nature, for example, are commonly used in moving glass sheets. In such operations, suction cups are typically connected to vacuum pumps and invested with translatory motion such that they can be made to approach and grip the outermost sheet of a stack. The outermost sheet, which may be at the top, bottom, front, or back of a stack, is then separated from the stack and translated toward the final destination by a distancing movement of the suction cups.
When the suction cups are to be detached from the sheet to allow further processing, the connection with the vacuum pumps is terminated, and the cups are vented to the atmosphere. Alternatively, the connection with the vacuum pumps can be terminated, and the cups subjected to a burst of compressed gas. The connections in question are controlled in most instances by means of pneumatically actuated valves, which are incorporated into the delivery system. For example, the use of three-way valves is conventional in this field.
In practice, these suction cups can leave traces of contact on the surface of a workpiece, which is typically a rigid sheet of material. For example, upon release of a suction device, suction cup shaped markings can be left on the handled surface. The suction device can also remove powders and/or other coatings that have been applied to the workpiece. Sheets of glass, for example, are often treated with protective powders (or “separators”), as discussed below. Removal of these powders by the suction device can have adverse effects on the glass. For example, the surface of the glass from which the powder is removed will subsequently be vulnerable to soiling, staining, and corrosion by water and/or other materials.
It is noted that moisture corrosion roughens the surface of glass. In fact, the longer water sits on a glass surface, the more the surface is typically affected. For example, as water sits on glass, alkali components are leached from the glass, thereby changing the pH of the water to alkaline. Thus, when water is allowed to remain on unprotected glass for a significant period of time, it tends to remove elements from the glass, particularly sodium ions. This process leaves a mark on the glass that, in some cases, can be so objectionable as to render the glass unsaleable.
As noted above, glass is typically treated with stain-retardants that prevent or minimize damage from moisture corrosion. For example, acidic buffers, such as adipic acid and maleic acid, are commonly sprayed onto the glass. These buffers are sometimes mixed into a powder which, as noted above, is generally referred to as a “separator”. The separator also provides protection during storage and transportation against abrasion between adjacent glass sheets in a stack.
As noted above, conventional pneumatic handling devices tend to remove some of the protective material from glass sheets during handling. For example, when glass is engaged by a pneumatic handling device, each suction cup acts somewhat like a vacuum cleaner. In effect, the protective material is vacuumed from the surface of the glass. Further, to effect disengagement of the suction cup from the glass, a short blast of pressurized air is sometimes applied. This blast of air can carry away even more of the protective powder. Finally, the portions of the suction cup that contact the glass tend to carry away protective material from the glass surface.
Often, it may not be immediately apparent that the protective powder has been displaced. In fact, suction cup marks may only appear after extended periods of storage. While these marks can be made visible by fogging the glass with steam, this procedure is so time-consuming as to be economically unfeasible. Even if no suction cup marks appear following storage, these marks may show up once the glass has been coated. To make matters worse, some suction cup marks only appear after the coated glass has been tempered. Of course, suction cup marks decrease the quality and value of glass. Moreover, depending on the severity of the marks, such damaged glass may be rejected in the marketplace.
It would be desirable to provide material handing devices and methods, particularly for glass sheets, which facilitate handling workpieces coated with protective material (e.g., powder) without removing such material during handling or replacing material upon release.